Many industrial processes and apparatus, as well as household devices, involve the separation of a liquid phase from another phase. In some instances, particularly when water is the phase present in minor amounts, chemical means may be used to remove the water from the other components. Such means for removing moisture, however, require the replacement and/or regeneration of the reagents used in the process. The reagents employed and the products formed frequently introduce complications relating to handling and disposal. Because of the concomitant cost and, in some instances, inconvenience and potential adverse effects from exposure to chemical reagents which are associated with such processes, physical methods and apparatus have been preferred to chemical means for removal of small amounts of a liquid phase from other phases.
A method of coalescing an immiscible liquid suspended in another phase and a coalescing device, frequently termed a "coalescer", have found
A method of coalescing an immiscible liquid suspended in another phase and a coalescing device, frequently termed a "coalescer", have found widespread use in removing liquid from both the gaseous phase, such as in aerosols, and from suspensions of one liquid in another liquid. Such devices are particularly effective where the volume of liquid removed is small in comparison to the volume of the phase from which it is removed. Typically, the equipment necessary to remove a liquid aerosol from a gas tends to be less complicated than that used to separate two liquid phases in which a first liquid phase is immiscible and suspended in a second liquid phase. This is generally true because in gas/liquid suspensions, gravitational effects tend to be more significant while surface energy, surface tension or interfacial tension effects tend to be less significant than with liquid/liquid suspensions.
The spectrum of applications where coalescers have been used to remove minor amounts of a first liquid phase, known as a "discontinuous phase" or "suspended phase", from a second liquid phase in which it is suspended, known as the "continuous phase" or "suspending phase", covers a considerable range of situations. For example, coalescers have been used most often to remove or separate small amounts of moisture from petroleum based fuels, including gasoline, diesel and aviation fuels, such as kerosene; to remove moisture from cleaning fluids; to separate oil from coolants and parts cleaners; to remove oil contamination found in natural bodies of water; to separate immiscible solvent systems used in extraction processes, etc.
Numerous mechanisms and models have been proposed to describe coalescence of a droplet of the discontinuous phase from the continuous phase and the ease or difficulty of separation of the immiscible phases. The factors which affect the coalescence process include the physical properties of the phases, such as density, viscosity, surface tension, shear rate, and interfacial tension (IFT). In addition, the properties of the system, such as drop size, curvature of the interface, temperature, concentration gradients and vibrations also affect coalescence significantly. While any or all of these factors may be significant in a particular situation, properties such as density, drop size and interfacial tension appear to be among the factors which are of most significance and often over which the least control can be exercised in difficult separations of two immiscible liquids. Thus, all other things being equal, where the densities of two liquids differ only slightly, separation becomes more difficult. This is also true of the interfacial tensions of the liquids involved. In those situations in which the droplets are greater than 10.mu. (primary emulsions) coalescence and separation is much easier to effect frequently with the discontinuous phase settling by gravity after coalescence to form a heterogeneous layer. When the droplets are smaller than 10.mu., particularly less than 1.mu. in diameter, secondary emulsions or secondary hazes result from which the discontinuous phase is much more difficult to coalesce. The latter frequently occurs where the emulsion has been formed by rigorous agitation or the inclusion of a surface active agent. Where emulsification to form the secondary haze occurs purely by mechanical means, coalescence may be accomplished much more readily by conventional coalescence methods and apparatus. Where the secondary haze results from surface active materials, which influence the interfacial tensions of the liquids, separation becomes more difficult.
The type of coalescer employed depends on the difficulty of separation or coalescence, as influenced by the factors identified above. Thus, in some situations, equipment may be very simple, such as those employing baffles as the coalescing-effective material, and range to more complex devices containing different types of packing. The type of fluids being separated frequently determines the packing used. Thus, both the shape of the packing material and its composition influence the efficiency of coalescence and separation. For example, the coalescing apparatus conventionally used to separate oil and water typically contain tubes, plates, disks, spears, rods, fibers or other internal structures designed to capture oil. Conventionally, glass has been the most often used packing material and while in some instances membranes have been employed in coalescers, as well as the types of packing listed above, fibers have been the preferred form of packing. Currently, glass fibers seem to have found the most widespread application in coalescers.
Although the development of different types of apparatus, methods and materials has advanced liquid-liquid separation technology, particularly in the separation of aqueous-organic liquids, some problems have proven more difficult to solve and advances in the technology related thereto have been less forthcoming. This is particularly true with regard to certain separation problems involving petroleum-based materials and water or aqueous solutions. A major source of problems in the petroleum industry has involved the separation of water or aqueous solutions from petroleum based fuels such as diesel fuel, kerosene and gasoline either at the site of processing or subsequent thereto. One reason for difficulty in separating the water or aqueous solution (present in minor amounts as a discontinuous phase) suspended in the fuel, for example gasoline (the continuous phase), is that reagents added during processing to remove unwanted components or surfactants or detergents added at the end of processing to assist in maintaining the cleanliness of fuel combusting equipment in which it is used reduces the IFTs of the aqueous and organic phases. This makes the discontinuous, aqueous phase more dispersed in the organic phase and, therefore, more difficult to separate by most methods employed to separate liquid phases. Both of these difficulties share something in common in that water-soluble components result in a reduction of the IFT, and the current technique widely used for separation in each case is only partially successful, although these techniques are somewhat different.
The former problem, the removal of corrosive materials, such as the addition of a corrosive material during processing, up until now, has relied upon conventional technology and its concomitant shortcomings. More specifically, the problem relates to the separation and removal of corrosive materials, such as aqueous acidic- or caustic-containing materials from petroleum fractions. Thus, in the processing of petroleum, various fractions may be treated with strong aqueous acid and/or alkali-containing solutions, such as in the removal of certain compounds which are undesirable in fuels, for example, sulfur-containing compounds. In such instances, the petroleum fraction may be initially washed with a strong aqueous acid and subsequently neutralized with an excess of alkali. In other instances, an acid solution alone may be used to treat the petroleum-based material. Removal of both acidic and caustic aqueous phases has been somewhat difficult from an efficiency and equipment standpoint, with removal of caustic aqueous phases proving most difficult, particularly with respect to the corrosive effect on the equipment employed. The use of coalescing and separating equipment generally employed in other separations of aqueous and organic phases has been obviated because of the nature of the materials involved. Thus, the IFTs of the liquid phases are very similar. Up until recently, separations using a conventional type of coalescer have not been achieved with IFTs lower than about 3 dynes/cm. In addition, the corrosive nature of some acid and most caustic solutions has ruled out the use of many materials which otherwise could be used to manufacture the processing equipment, including many metals.
Since effective coalescers which are resistant to corrosive materials, particularly, caustic substances have been unavailable, heretofore, the most effective technology available to the petroleum industry to remove caustic from hydrocarbon fuels, has been for decades, and continues to be, a sand bed filter. While being referred to as filters, and functioning at least in part as such, these devices may also be loosely categorized as functioning as coalescers. They are, however, different in most respects from coalescers currently being used in most other applications. These are massive filters having volumes on the order of about 5000 to about 7000 cubic feet. Not only are such filters extremely large, but they also are expensive to build and maintain. It is generally not practical to attempt to clean the sand and both the sand and some other components of the filter are routinely removed and disposed of. Since many of the components cannot withstand the long-term exposure to the caustic environment, occasionally entire sand bed filter systems must be replaced at a large capital cost.
In view of these problems, methods and apparatus have been sought which will allow the separation of aqueous and organic liquid phases where the differences between the IFTs of each phase is very low. A method and apparatus have also been sought to separate a corrosive aqueous phase, particularly a caustic-containing aqueous phase, from a substantially water-insoluble phase which, when compared to currently used techniques in the petroleum industry, permits the use of apparatus having a smaller volume, equivalent or greater efficiency, equivalent or lower initial capital investment and lower labor and replacement related maintenance costs.
Since the separation of aqueous solutions used in processing steps in the petroleum industry are frequently performed at or involve petroleum fractions at elevated temperatures, the method and apparatus should be capable of performing effectively at elevated temperatures.